Anyway, no game lasts forever, unless you're counting times where you pick up some old game for a week, 2 years after your last play-through. This may be one of those but that doesn't mean it "lasts forever".
I'm at 90 hours and I still haven't finished the MQ with any character. Without any mods, I think I won't be playing anything else for a few more weeks. Then it may start to go on and off until I squeeze every little drop of entertainment out of it.
About the Radiant Story system, I'm playing my 3rd character and I haven't found enough instances of this to really make a difference. Even though that may help, the game is very re-playable regardless of it, in my opinion.